1. Field of the Invention
The present invention relates to image recording systems of the type in which electrographic toner images are formed on a recording element. More particularly, it relates to a method and apparatus for controlling the transmission density of such images without using transmission densitometry techniques.
2. Discussion of the Prior Art
In the commonly assigned U.S. patent application Ser. No. 673,509, filed on Nov. 30, 1990 in the name of DeBoer et al., there is disclosed an image-recording apparatus in which an intensity-modulated laser beam scans a uniform layer of pigmented thermal plastic particles to cause the irradiated particles to adhere to an underlying thermoplastic substrate in an image configuration. Thereafter, the non-exposed particles are removed from the substrate surface, leaving behind a visible toner image of the information being recorded.
In the above image recording system, the thermoplastic substrate on which image recording is carried out is backed by a grounded electrode, and lay-down of the uniform layer of toner particles is effected by an electrically biased magnetic brush. The level of bias on the brush controls the toner mass lay-down, i.e., "mass per unit area" and, hence, the transmission density of the toner layer For certain applications which make use of the above-mentioned recording process, it is desirable to maintain the maximum transmission density within certain prescribed limits notwithstanding relatively large changes in ambient environmental conditions. When the underlying substrate is transparent to optical radiation, it is a relatively simple matter to monitor, via transmission densitometry techniques, the transmission density of the toner layer or a "test patch" and, by a conventional feedback scheme, to vary the brush bias in order to maintain or achieve a desired transmission density. However, where the underlying substrate is opaque, another approach must be found.
An alternative approach to directly measuring the transmission density of the toner layer is to monitor the reflection density thereof. The reflection density is readily correlated with transmission density at low density levels. But this indirect approach transmission density detection is not useful in those cases where the toner mass lay-down is such that the corresponding reflection density is saturated, or where the toner support does not provide sufficient contrast with the toner material. In some commercial imaging applications, there is a need to provide substantially higher image densities than those detectable by reflection densitometry techniques, and to record such images on relatively non-reflective substrates. Thus, there is a need for a method and apparatus for indirectly determining the transmission density of a relatively dense layer of toner particles on a light-absorbing support.